The present invention relates to an electrically conductive resinous composition, a fuel cell separator made of said electrically conductive resinous composition and a process for production thereof, and a polymer electrolyte fuel cell consisting of a plurality of said fuel cells in which all or part of the separators are said ones.
A fuel cell is an apparatus which has a pair of electrodes, with an electrolyte interposed between them, one electrode being supplied with a fuel and the other electrode being supplied with an oxidizing agent, for the oxidation of fuel to take place electrochemically in the cell, thereby achieving direct conversion from chemical energy into electrical energy. Fuel cells fall into several types according to the electrolyte used therein. There has recently been developed a polymer electrolyte fuel cell which employs a polymer electrolyte membrane as the electrolyte. This fuel cell is attracting attention because of its high output.
The polymer electrolyte fuel cell consists of two fuel cell separators 1 and one polymer electrolyte membrane 2 and two gas diffusion electrodes 3 which are held between the separators, each separator having a plurality of ribs 1a on both sides thereof, as shown in FIG. 1. Tens to hundreds of such fuel cells (as unit cells) are connected together to form a stack of fuel cells.
The polymer electrolyte fuel cell operates in such a way that the electrode for fuel is supplied with hydrogen gas (fluid) and the electrode for oxidizing agent is supplied with oxygen gas (fluid), thereby providing external circuits with current. The following reactions (1) and (2) take place on the two electrodes. On electrode for fuel:
H2xe2x86x922H++2exe2x88x92xe2x80x83xe2x80x83(1) 
On electrode for oxidizing agent:
2H++2exe2x88x92+xc2xdO2xe2x86x92H2Oxe2x80x83xe2x80x83(2) 
Overall reaction:
H2+xc2xdO2xe2x86x92H2O 
In other words, on the electrode for fuel, hydrogen (H2) changes into protons (H+), and these protons move to the electrode for oxidizing agent through the polymer electrolyte membrane. On the electrode for oxidizing agent, the protons react with oxygen (O2) to give rise to water (H2O). Therefore, the operation of polymer electrolyte fuel cell necessitates the supply and discharge of reaction gas and the output of current. The polymer electrolyte fuel cell is usually expected to work in a wet atmosphere at room temperature up to 120xc2x0 C. Therefore, reaction on the electrode for oxidizing agent gives rise to liquid water, which has to be discharged adequately.
The fuel cell mentioned above includes separators as its constituent parts. As shown in FIGS. 2A and 2B, the separator is a thin platy body having a plurality of ribs 1a on both sides thereof and a plurality of gas feed grooves 4 on one side or both sides thereof. The separators permit fuel gas, oxidizing agent gas, and cooling water to flow through the fuel cell without mixing together. They also lead electric energy or heat evolved in the fuel cell to the outside. The fuel cell separators are required to have good gas barrier properties, conductivity, corrosion resistance, mechanical strength, and impact resistance. Separators should be strong enough to withstand tightening by bolts and nuts at the time of cell assembling. Impact resistance is necessary particularly when fuel cells are used for automobiles.
Separators for polymer electrolyte fuel cell are conventionally made of a carbonaceous composite material which contains a thermoplastic resin or thermosetting resin as a binding agent from the standpoint of productivity and production cost. The use of a thermosetting resin (such as phenolic resin) as a binding gent is disclosed in Japanese Patent Laid-open No. Sho 59-26907, and the use of a thermoplastic resin (such as polypropylene and nylon) as a binding agent is disclosed in Japanese Patent Laid-open No. Sho 56-116277.
The separator made of the above-mentioned carbonaceous composite material is superior in productivity and production cost to the conventional one produced by machining graphite plate; however, it is not necessarily satisfactory in performance such as mechanical strength, chemical resistance, and gas permeability.
The present invention was completed in view of the foregoing. It is an object of the present invention to provide an electrically conductive resinous composition suitable for mass production of fuel cell separators superior in electrical conductivity, mechanical properties, chemical resistance, gas impermeability, and moldability. It is another object of the present invention to provide a fuel cell separator produced from said electrically conductive resinous composition and a process for production thereof. It is another object of the present invention to provide a polymer electrolyte fuel cell in which all or part of separators are said ones.
In order to achieve the above-mentioned object, the present inventors carried out extensive studies in search of an electrically conductive resinous composition to be made into a fuel cell separator with low resistivity and high mechanical strength. As the result, it was found that the object is achieved with an electrically conductive resinous composition which is composed of an electrically conductive carbon powder and a binding agent which is a mixture of a thermoplastic resin and a carbodiimide compound. This electrically conductive resinous composition affords a fuel cell separator having low resistivity and improved mechanical strength, gas barrier properties, and chemical resistance. It also solves the problem with high-temperature durability which is encountered in the conventional binding agent prepared from a thermoplastic or thermosetting resin alone. This finding is the basis of the present invention.
According to the present invention, the polymer electrolyte fuel cell is constructed such that all or part of its separators are those of the present invention. The fuel cell separator has good electrical conductivity, mechanical strength, chemical resistance, gas barrier properties, and moldability. By virtue of these characteristics, the stack of fuel cells maintains a high operating efficiency (with a small decrease in output) even after continuous operation for a long time. It is suitable for use as a mobile power source for cars and small ships.
The present invention is directed to an electrically conductive resinous composition, a fuel cell separator and production thereof, and a polymer electrolyte fuel cell, as explained in the following.
The first aspect of the present invention covers an electrically conductive resinous composition composed mainly of an electrically conductive carbon powder and a binding agent, wherein the binding agent is a mixture of a thermoplastic resin and a carbodiimide compound.
The second aspect of the present invention covers an electrically conductive resinous composition as defined in the first aspect, wherein the mixture consists of 100 parts by mass of the thermoplastic resin and 0.001-50 parts by mass of the carbodiimide.
The third aspect of the present invention covers an electrically conductive resinous composition as defined in the first or second aspect, wherein the electrically conductive carbon powder is one which has a mean particle diameter of 10 to 500 xcexcm, and the amount of the electrically conductive carbon powder is 100-1000 parts by mass for 100 parts by mass of the thermoplastic resin.
The fourth aspect of the present invention covers a fuel cell separator which is molded from the electrically conductive resinous composition defined in any of the first to third aspects, wherein the fuel cell separator has on one side or both sides thereof grooves through which an oxidizing gas or fuel gas is supplied, and the fuel cell separator also has a resistivity not higher than 200 mxcexa9xc2x7cm.
The fifth aspect of the present invention covers a process for producing a fuel cell separator from an electrically conductive resinous composition composed mainly of an electrically conductive carbon powder and a binding agent (which is a mixture of a thermoplastic resin and a carbodiimide compound), the fuel cell separator having on one side or both sides thereof grooves through which an oxidizing gas or fuel gas is supplied, the process comprising the steps of injection-molding a mixture of 100 parts by mass of the thermoplastic resin, 0.001-50 parts by mass of the carbodiimide compound, and 100-1000 parts by mass of the electrically conductive carbon powder.
The sixth aspect of the present invention covers a polymer electrolyte fuel cell consisting of a plurality of unit cells connected together, each unit cell consisting of a pair of electrodes holding a polymer electrolyte membrane between them and a pair of separators holding the electrodes between them, the separator having passages molded thereon through which gas is supplied and discharged, characterized in that all or part of the separators in the fuel cells are those which are defined in the fourth aspect.
The seventh aspect of the present invention covers a polymer electrolyte fuel cell as defined in the sixth aspect, which retains no less than 85% of its initial output after continuous operation for 200-500 hours.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.